The proposed research i Project 2 of the Mid-America Adolescent Sexually Transmitted Diseases Clinical Research Center (MASTD CRC). A unifying thematic question for the research is "Why are some at-risk adolescents infected by sexually transmitted organisms and others aren't I?" The research proposed in project 2 is designed to examine relationships among Chlamydia trachomatis omp1 genotype, aspects of humoral immunity, and persistent infections and reinfections among women in middle adolescence. Among sexually active age groups, adolescents are at highest risk for acquiring chlamydial infection, and genital shedding of chlamydia is most intense among adolescents. The major complication of chlamydial genital infection, pelvic inflammatory disease,is also most common among adolescents, and can lead to involuntary infertility and ectopic pregnancy. In turn, these sequelae occur must frequently among women with either persistent infection or reinfection. The major goal of the proposed research is to make an unprecedented examination infection and reinfection in adolescent women using combined biological markers, and detailed behavioral and epidemiological data. These data will be simultaneously obtained through three 12 week periods of intensive data collection during a 27 month study period. Aim 1 will determine the degree of genetic diversity at the omp1 locus by amplifying and sequencing the gene encoding the most variable of chlamydial proteins. the major outer membrane protein (MOMP). These genotyping data are fundamental to the analyses of persistence versus reinfection in Aim 2 and immunity on exposure in Aim 3. Aim 2 is focused on developing an algorithm for distinguishing between persistent infection and reinfection in adolescents during the intensive data collection periods using combined biological, behavioral and epidemiological data. Aim 3 is to determine whether the acquisition of new infection in adolescent women after sexual exposure to an infected partner is reduced by neutralizing and epitope-specific antibodies in serum and secretions prior to the contact. The studies are an integrated clinical and basic science approach to factors influencing persistent infection and reinfection.